1. Field of the Invention
The present invention generally relates to a power steering system for vehicles. More particularly, the invention relates to an electric power steering system for vehicles which produces auxiliary steering torque by means of a steering servo device using an electric motor.
2. Description of Relevant Art
In view of problems in hydraulic type power steering systems such as that the structure thereof is complicated, in recent years a variety of electric type power steering systems for vehicles have been proposed.
As one of them, an example of an analogue control type is disclosed in Japanese Patent Application Lay-Open Print No. 59-70257, laid open Apr. 20, 1984.
According to this Japanese Lay-Open Print, an electric power steering system for vehicles includes a steering servo device having as its power source an electric motor and an analogue type control circuit for controlling the servo device in dependence on a steering torque signal from a detection circuit adapted to detect steering torque acting on a steering wheel, and by making the control circuit cause the electric motor to produce auxiliary torque it is devised to render the steering force light. The analogue type control circuit is adapted to have an armature voltage PWM-controlled to be impressed to the electric motor, with such a polarity that, while the steering wheel is operated to rotate in either direction thereof, causes the electric motor to rotate in a direction corresponding to the steering direction; there being fed back a signal corresponding to an armature current.
However, not limitting to the power steering system according to the aforementioned Japanese Lay-Open Print, in electric power steering systems in general, following the rotating operation of a steering wheel, an electric motor and other rotary members cooperating therewith, such as a reduction gear for example, are caused to rotate. Therefore, in cases where the steering speed of the steering wheel is changed from high to low speed or from low to high speed within a relatively short time, there arises such a problem that the steering operation receives effects of the moment of inertia, that is, inertial torque of such rotary members as described above.
In this respect, in the power steering system according to the aforementioned Japanese Lay-Open Print, with an eye to the fact that steering torque suddenly changes when the steering wheel is suddenly steered back in a steering operation thereof in either direction, the electric motor is adapted to be braked in accordance with variations of torque. In other words, when the steering wheel is suddenly steered in opposite direction, in accordance with an associated torque variation a braking is made to the electric motor by applying thereto a pulse signal with such a polarity that causes the electric motor to rotate in a direction corresponding to the reverse steering direction. However, such pulse signals for braking the electric motor are applied thereto by making use of time intervals between the PWM-controlled signals which are impressed to the electric motor to cause a forward rotation thereof. As a result, in the power steering system according to the aforementioned Japanese Lay-Open Print it may be difficult to favorably avoid such a problem as aforementioned of effects of the moment of inertia or inertial torque of the electric motor and rotary members cooperating therewith, in addition to that there is a problem concerning the durability of electric motor and besides electric power loss is still large.
On the other hand, in recent years, due to the advantage that complicated control functions can be achieved with a relatively simple system constitution, microcomputer systems fundamentally adapted for digital signal processes tend to be utilized as control means of various working devices.
In this respect, it would be desirous to utilize a microcomputer system in the form of a control device for such steering systems as aforementioned.
However, since in general microcomputer systems are unable to concurrently read many input signals and besides are adapted to execute signal processes thereof in a sequential manner depending on a system clock pulse, in attempts to utilize a microcomputer system to achieve control functions similar to those of an analogue control circuit of a conventional electric power steering system of the type described, for example, there is an attendant restriction or problem such that a predetermined process time is required.
In this respect, in cases where a feedback control is performed by using a microcomputer system, it is needed to repeat a feedback loop so many times and such a problem as described above becomes remarkable.
For such reasons, in cases where a microcomputer system is utilized as a control device of an electric power steering system, there is an attendant possibility such that an electric motor for producing auxiliary torque can not be so controlled as to sufficiently follow up the speed of steering operation, thus resulting in the possibility of unsuccessful realization of optimum steering feeling.
In view of such a problem in conventional electric power steering systems as descrived, the present invention has been achieved to effectively solve same, and particularly, the invention has been achieved to overcome, even in cases where for the solution of such problem a microcomputer system is utilized as a control device, the aforementioned restriction or problem attending thereon.